**Background:** Sauerkraut is a traditional lactic acid-fermented cabbage product widely consumed in northeastern China. While lactic acid bacteria (LAB) are known to drive fermentation, traditional methods often yield inconsistent quality and limited flavor profiles. The emergence of starter culture-fermented sauerkraut products aims to improve nutritional value, flavor, and safety. However, systematic comparisons of microbial communities and metabolite profiles between naturally fermented and starter culture-fermented sauerkraut remain limited. This study investigates how different microbial addition strategies affect the microbial composition, nonvolatile and volatile metabolites, and their interrelationships in commercial northeast Chinese sauerkraut.
**Methods:** Four commercial sauerkraut types were purchased from Harbin, China: LLS (L. paracasei added), HX (L. plantarum added), QM (L. plantarum and L. acidophilus added), and ZLL (naturally fermented, no starter culture). Physicochemical properties (pH, total acids, reducing sugars, salt content) were measured. Microbial diversity was assessed via 16S rRNA high-throughput sequencing targeting the V3-V4 region. Nonvolatile compounds (26 amino acids, 11 organic acids) were quantified using UPLC-Q-TOF/MS. Volatile organic compounds were analyzed by headspace gas chromatography-ion mobility spectrometry (HS-GC-IMS). Associations between bacterial taxa and metabolites were evaluated using O2PLS modeling and Spearman correlation analysis.
**Key Results:** A total of 605,269 high-quality sequences were generated, yielding 428 OTUs at 97% similarity. At the phylum level, Firmicutes and Proteobacteria dominated all samples, with Firmicutes relative abundance ranging from 83.3% to 99.9%. Naturally fermented ZLL had the highest Firmicutes abundance (99.7%), while LLS had the lowest (83.3%), indicating that added strains inhibited Firmicutes production. At the genus level, LLS showed the highest microbial diversity (Shannon/Simpson indices), while QM and ZLL had lower diversities. LLS was dominated by Lactobacillus (73%), Chlorobacillus (10%), and Lactococcus (3.3%). LEfSe analysis identified 6 biomarkers: Leuconostoc, norank_f__norank_o__Chloroplast, and Lactococcus for LLS; Kosakonia and Pediococcus for HX; Lactobacillus for ZLL; no biomarkers were identified for QM.
A total of 26 amino acids were detected. ZLL had the highest total amino acid content, followed by QM. Phenylalanine, leucine, isoleucine, tryptophan, histidine, and proline were most abundant. Eleven organic acids were identified; lactic acid was significantly higher in mixed-strain fermented sauerkraut (QM) than in single-strain or naturally fermented types. Succinic acid and pyroglutamic acid together accounted for 61.5% and 34.3% of total organic acids, respectively. GC-IMS identified 88 volatile compounds: 24 alcohols, 17 aldehydes, 11 esters, 9 ketones, 6 acids, and 21 other compounds. Alcohols, esters, and acids were more abundant in bacterially fermented sauerkraut. LLS had the highest number of alcohols (18). PCA of volatile compounds showed clear separation among the four types, with PC1 and PC2 explaining 78.1% of variance. Characteristic volatiles included: ethyl-2-methylpropanoate, 1-pentanol, E-2-octenal, 1,8-cineole, and isobutyl acetate for LLS; α-pinene, phenylacetaldehyde, and 2-cyclohexen-1-one for HX; N-diethylethanamine and benzaldehyde trans-2-heptenal for QM; and methylpyrazine, z-3-hexen-ol, heptanol, 2-octanol, and 2-methylpropanoic acid for ZLL.
O2PLS modeling (R² = 0.99, Q² = 0.78 for both amino acids and organic acids) revealed strong correlations between bacterial taxa and metabolites. Cystine and tyrosine were significantly correlated with 18 genera. Ascorbic acid and acetic acid were significantly correlated with 13 bacterial taxa. Lactobacillus, Pediococcus, and Lactococcus showed strong positive correlations with alcohols and esters.
**Clinical Implications:** While this study does not directly address clinical outcomes, it provides foundational data for improving fermented food quality through starter culture selection. Understanding microbiota-metabolite correlations can guide the development of sauerkraut with enhanced nutritional profiles (e.g., higher amino acid content) and improved flavor characteristics. This may have indirect implications for dietary interventions involving fermented foods, though no health claims are directly supported by this study. The findings support industrial quality control and brand differentiation in the sauerkraut market.